1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplex Access (OFDMA)/Code Division Multiple Access (CDMA) mobile communication system. More particularly, the present invention relates to an apparatus and a method for efficiently transmitting a broadcasting channel by means of cyclic delay diversity.
2. Description of the Related Art
Generally, an OFDM/CDMA mobile communication system, which is proper for high speed data transmission through a radio channel, uses a plurality of mutually orthogonal carriers.
An OFDM scheme has been adopted as a wireless standard scheme in such fields as Digital Audio Broadcasting (DAB), and Digital Video Broadcasting-Terrestrial (DVB-T). In addition, the OFDM scheme is used in the Institute of Electrical and Electronics Engineers (IEEE) 802.11a Local Area Network (LAN) standard, and the IEEE 802.11a Metropolitan Area Network (MAN) standard. Accordingly, an OFDM scheme is being discussed as a representative scheme in the 4G generation mobile communication and the next generation mobile communication systems.
Transmission of an OFDM symbol is performed by coding the symbol and the transmission is influenced by the previous symbol during transmission through a multi-path channel. In order to prevent interference between OFDM symbols, guard intervals longer than a maximum delay spread of a channel are inserted between continuous symbols. Accordingly, an OFDM symbol period is a sum of an effective symbol period during which data is actually transmitted and a guard interval. Further, a reception side removes the guard interval, extracts data during the effective symbol period, and performs a demodulation of the data. Herein, a guard interval obtained by copying signals of a last portion in an effective symbol interval is inserted in order to prevent orthogonality from being destroyed which may occur due to a delay of a sub-carrier. The guard interval obtained by copying signals of a last portion in an effective symbol interval will be referred to as a Cyclic Prefix (CP).
FIG. 1 is a block diagram of a conventional OFDM system supporting a multi-carrier delay diversity modulation.
Referring to FIG. 1, a coder 100 codes an input data sequence and outputs the coded data sequence Xk to a serial-to-parallel converter 105. The serial-to-parallel converter 105 divides the data sequence into N sample data and outputs the divided sample data in parallel to an Inverse Fast Fourier Transform (IFFT) unit 110. The IFFT unit 110 inputs the N sample data output from the serial-to-parallel converter 105, performs an IFFT for the N samples of data, and outputs N number of OFDM data samples in parallel to a parallel-to-serial converter 115. The parallel-to-serial converter 115 receives the OFDM sample data output from the IFFT unit 110, converts the OFDM sample data into serial data, and outputs the serial data. The guard interval inserter 120 copies a G number of last OFDM data samples from the OFDM sample data in the OFDM symbol constructed from the N number of OFDM data samples, inserts the copied data sample as a guard interval prior to the OFDM symbol, and outputs the guard interval and the OFDM symbol. Herein, the OFDM symbol including the guard interval will be referred to as an OFDM transmission symbol. Accordingly, an antenna unit 125 receives the OFDM transmission symbol output from the guard interval inserter 120, converts the OFDM transmission symbol into analog signals, which are OFDM signals, and outputs the OFDM signals.
Herein, a base station generates different multi-path channels from the OFDM signal by using a predetermined delay, and this will be referred to as a multi-carrier delay diversity modulation scheme. According to the multi-carrier delay diversity modulation scheme, when the base station transmits signals through multiple antennas, each antenna delays the signals by a predetermined amount of time and then transmits the delayed signals. This scheme is different from a Space-Time Block Coding (STBC) scheme and a Space-Time Trellis Coding (STTC) scheme which improves transmit diversity using multiple antennas.
The multi-carrier delay diversity modulation scheme may flexibly perform a space-time coding regardless of the number of transmit antennas, as compared with the STBC scheme. Further, the multi-carrier delay diversity modulation scheme has low complexity and may operate without limitation in the number of transmit antennas, as compared with the STTC scheme.
In other words, the multi-carrier delay diversity modulation scheme may perform the space-time coding without being limited by the number of transmit antennas, and may maximize the diversity performance without greatly changing an existing transmission or reception scheme.
Further, the multi-carrier delay diversity modulation scheme may use an existing reception method for reception.
That is, a codeword sequence (x0, x1 . . . , xN-1) having a block length of N output from the serial-to-parallel converter 105 is modulated into N sub-carriers by the IFFT unit 110, and is then multi-carrier-modulated by Equation 1 below.
                                          X            n                    =                                    1                              N                                      ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  k                                ⁢                                  ⅇ                                      j2π                    ⁢                                                                                  ⁢                                          kn                      /                      N                                                                                                          ,                  n          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    Equation        ⁢                                  ⁢        1            
In Equation 1, k represents a sub-carrier index, n represents a time domain sample index, Xk represents a frequency domain symbol sequence, and Xn represents a time domain symbol sequence.
Accordingly, in the multi-carrier delay diversity modulation scheme as illustrated in FIG. 1, signals transmitted through M transmit antennas may be expressed by a tapped delay line having a length of M−1. A delay interval is identical to a sample interval T of the sequence {Xn}. Further, a codeword may be expressed by Equation 2 below.
                    C        =                  (                                                                      x                  0                                                                              x                  1                                                            ⋯                                                              x                                      N                    -                    1                                                                                                                        x                                      N                    -                    1                                                                                                x                  0                                                            ⋯                                                              x                                      N                    -                    2                                                                                                      ⋮                                            ⋮                                            ⋰                                            ⋮                                                                                      x                                      N                    -                    M                    +                    1                                                                                                x                                      N                    -                    M                    +                    2                                                                              ⋯                                                              x                                      N                    -                    M                                                                                )                                    Equation        ⁢                                  ⁢        2            
In Equation 2, an Mth antenna unit 155 transmits signals (xN−M, xN−M−1, . . . ,xN−M+1) obtained by cyclically shifting the sequence (x0, x1, . . . , xN−1) by M times. The codeword C will be referred to as a cyclic delay codeword because of the cyclic delay shifting scheme as described above.
According to the multi-carrier delay diversity modulation scheme, a second guard interval inserter 140 receives the N number of OFDM data samples delayed by T, copies a G number of OFDM data samples from the last OFDM sample data of the OFDM sample data, inserts the copied G number of data samples as a guard interval prior to the OFDM symbol, and outputs the guard interval and OFDM symbol. Accordingly, a second antenna unit 145 transmits OFDM signals corresponding to xN−1, x1, . . . ,xN−2. Further, an Mth guard interval inserter 150 receives signals obtained by cyclically delaying once again signals already cyclically delayed (M−1) times, that is, signals (xN−M, xN−M−1, . . . , xN−M+1) obtained by cyclically shifting the sequence (x0, x1, . . . , xN−1) M times. The Mth guard interval inserter 150 copies G number of OFDM data samples, inserts the guard interval into the copied OFDM sample data, and outputs the OFDM sample data. An Mth antenna unit 155 transmits OFDM signals corresponding to xN−M−1, xN−M−2, . . . ,xN−M.
In the prior art, the OFDM signals delayed by T were cyclically delayed, thereby obtaining an additional frequency diversity gain. In other words, when the delay diversity is employed, a scheme of cyclically delaying only one sample according to each antenna has been used.
When the base station transmits a broadcasting channel, it is necessary to consider if delayed signals delaying only one sample as described above may cause a transmit diversity gain to a user terminal in a different cell. This is because a transmit diversity gain may occur or interference may increase due to a system characteristic such as the length of a guard interference interval, the cell type, the number of sectors, or the beam pattern. That is, it is necessary to provide a transmission scheme capable of transmitting OFDM signals more efficiently in a blanket area where reception of the broadcasting channel is poor, a hot spot area where user terminals are crowded, or an outskirt area where user terminals appear occasionally.